Given:

Atomic Mass of First Isotope of Chlorine = 34.9689g

Natural Abundance of First Isotope of Chlorine, P_a1 = 75.77%

= 0.7577

Total Atomic Mass of 1^st Isotope, M_a1 = 34.9689×0.7577

= 24.4959g

Atomic Mass of Second Isotope of Chlorine = 36.9659g

Natural Abundance of Second Isotope of Chlorine, P_a2 = 24.23%

= 0.2423

Total Atomic Mass of 2^nd Isotope, M_a2 = 36.9659×0.2423

= 8.9568g

So Average Atomic Mass of Chlorine =

→

→ 35.4617

≈35.462g

Therefore, the average atomic mass of Chlorine is 35.462g.